There's loads of difference. If signed you have the option to sell them in January if they are razzling and someone wants them.
If loaned you cannot play them against there parent club and you might be obliged to give them game time. At the end of the loan you are at the mercy of the parent club where they go, you cant offer them a contract. We had Harry Clarke on loan from Arsenal and he was loaned to Hibs for whatever reason, totally out with the clubs control, if he had been a signing we would have had the option to offer him a competitive contract.